What is the largest a smartphone screen size can be before it can't be called a smartphone anymore? (it would be a tablet)

EDIT: I know about the Dell Streak. They are calling it a tablet, not a smartphone. So what I am asking is, what's the screen size to qualify a device as a tablet versus a smartphone? (4.4", 4.5", 4.6", 4.7", 4.8", 4.9", 5"???) The EVO and Droid X have a 4.3" screen and the Dell Streak (a so called tablet) has a 5" screen, so somewhere in between 4.3 and 5 is the deciding size. What size is it???